In a multi-carrier transmission, if there is a delay wave which is over a guard interval (GI), an inter symbol interference (ISI) or an inter carrier interference (ICI) occurs. If a previous symbol enters an FFT (Fast Fourier Transform) interval, the inter symbol interference occurs. If breaks (in other words, a discontinuous interval) of the symbol enter the FFT interval, the inter carrier interference occurs.
FIG. 28 is a diagram showing signals which are transmitted from a wireless transmission device to a wireless reception device via a multi-pass environment. Here, the horizontal axis is time. Symbols s1 to s4 indicate signals which are transmitted from the wireless transmission device to the wireless reception device via a multi-pass environment, and are transmitted via four multi-pass. In previous symbols, guard intervals (GI) which are copied latter of the symbols are added
A first signal s1 from the above indicates a direct wave, and a second signal s2 indicates a delay wave which occurs a delay t1 which is shorter than the guard interval (GI). The direct wave and the delay wave are also called arrival waves.
A third signal s3 and a fourth signal s4 are delay signals which occur delays t2 and t3 which are over guard intervals (GI).
The diagonal line portion of previous third and fourth delay waves s3 and s4 indicate portions where the previous symbol of predetermined symbol enters the FFT interval of the predetermined symbol. The interval t4 indicates the FFT interval of the predetermined symbol. The diagonal line portions become the ISI elements. The ISI element occurs a characteristics depletion of demodulation, because the ISI element is an interference element. In a third delay wave of signal s3 and a fourth delay wave of signal s4, symbol's breaks enter the interval t4, and this occurs the ICI.
FIG. 29(a) and FIG. 29(b) are diagrams showing a state that sub-carriers are orthogonal, and interference occurs between sub-carriers by the ICI, in transmission and received signals of multi-carrier system. FIG. 29(a) shows a state that the ICI does not occur and interference does not occur between sub-carriers. FIG. 29(b) shows a state that the interference occurs between sub-carriers by the ICI.
When the delay wave which is over the guard interval (GI), as shown in FIG. 29(a), if the dot line portion is looked at, a certain one sub-carrier element is included, and another sub-carrier element is not included. This state is that the orthogonality is maintained. In normal multi-carrier communication, the demodulation is performed in this state.
In contrast to this, if the delay wave which is over the guard interval (GI) exists, as shown in FIG. 29(b), when the frequency shown dot line portion is looked at, a contiguous sub-carrier element and not a predetermined sub-carrier element is included, and interference occurs. This state is that orthogonality is not maintained between sub-carriers. ICI becomes a reason of characteristics depletion.
Patent Document 1 discloses one method to improve the characteristics depletion by ISI and ICI, when delay waves are over the guard interval (GI). In this prior art, after performing a demodulation operation once, using an error correction result (output of a MAP decoder), after generating copy signal (replica signal) except predetermined sub-carriers which include the ISI element and ICI element, then, for the copy signal which is removed from the received signal, the demodulation operation is performed again, and the characteristics of the ISI and the ICI are improved.
On the other hand, as the system which combines the multi-carrier transmission system and the CDM (Code Division Multiplexing) system, the MC-CDM (Multi Carrier Code Division Multiplexing) system has been suggested.
FIG. 30(a) and FIG. 30(b) are diagrams showing a relationship between orthogonal codes corresponding to the sub-carrier of the MC-CDMA system, and each sub-carrier. In these figures, the horizontal axis indicates frequency. In FIG. 30(a), for example, eight sub-carriers of the MC-CDM system are indicated. In FIG. 30(b), as orthogonal codes which corresponding to each sub-carrier, three types (C8,1, C8,2 and C8,7) are shown. Here, C8,1=(1, 1, 1, 1, 1, 1, 1, 1), C8,2=(1, 1, 1, 1, −1, −1, −1, −1) and C8,7=(1, −1, −1, 1, 1, −1, −1, 1). By multiplexing the three types of orthogonal codes to the data, it is possible to communicate with multiplexing three data sequences at same time and using the same frequency. This is one feature of the MC-CDM system.
Three types of orthogonal codes C8,1, C8,2, and C8,7 are orthogonal codes which have a cycle of eight entirely. By multiplexing between one cycles, in the orthogonal codes, it is possible to separate the data.
FIG. 31(a) and FIG. 31(b) are diagrams showing C′8,1, C′8,2, C′8,7, C″8,1, C″8,2 and C″8,7 when the signals of the MC-CDMA system are transmitted, and then received by the wireless reception device. FIG. 31(a) shows a state when the frequency change is not occurred in the cycle of the orthogonal codes. In this case, the de-spreading is performed with C8,1. In other words, an inner product with C8,1 is calculated. If all values in the SFfreq are added, C′8,1 is 4, C′8,2 and C′8,7 are zero. A state like this is called the orthogonality between codes is maintained.
In contrast this, in FIG. 31(b), when a frequency change exists in the cycle of the orthogonal codes, if inverse diffusion is performed with C8,1, C″8,1 is 5, C″8,2 is 3, and C″8,7 is zero. In other words, the interference element exists between C″8,1 and C″8,2, and the orthogonality between codes is not maintained. Like this, if the frequency change of channels is rapid (changes early along in the frequency direction), in the MC-CDMA system, the multi code interference becomes a reason of characteristics depletion.
Patent Document 2 and Non-Patent Document 1 disclose one method of improving characteristics depletion by breaking of the orthogonality between the codes. In this prior arts, the difference between the downlink and uplink exists. However, both of them use the data after the error correction or the inverse diffusion is performed, and the characteristics is improved by removing other than the predetermined signal, to remove multi code interference by code multiplexing during MC-CDMA communication.    Patent Document 1: Japanese Unexamined Patent Publication, First Publication No. 2004-221702    Patent Document 2: Japanese Unexamined Patent Publication, First Publication No. 2005-198223    Non-Patent Document 1: “Downlink Transmission of Broadband OFCDM Systems-Part I: Hybrid Detection”, Zhou, Y.; Wang, J.; Sawahashi, M. Page(s): 718-729, IEEE Transactions on Communication (Vol. 53, Issue 4)